The invention relates to apparatus for in-situ dressing of threaded type grinding wheels used in gear grinding machines.
The art field embracing gear grinding machinery is typified substantially by two types of machine, the designs of which differ according to the adopted grinding technique.
In a first type of machine, used to implement the NILES method, the work is offered to a bevelled disk type grinding wheel (two conical surfaces disposed coaxially and symmetrically back to back), whereas in the second type, the tooth or thread profile is generated (REISHAUER method).
Machines falling into the first mentioned category are especially suitable for grinding spur or helical teeth on cylindrical gears, and relatively simple in embodiment.
Machines of the second type referred to are designed to generate helicoid tooth flanks such as those, for example, of a worm, and make use of a grinding wheel exhibiting a threaded profile which meshes with the work during the grinding operation, such that both the wheel and the work describe movements typical of gear cutting (e.g. as in six-axis machines), with the tooth flank being generated as an envelope of straight lines rolling on the base circle.
A threaded grinding wheel of the type used for the REISHAUER process is usually dressed without the wheel being removed from its support in the grinding machine; this ensures speed and accuracy, and avoids any error that would occur were it to be shuttled back and forth continually between the grinder and a separate dresser.
A threaded grinding wheel is dressed in exactly the same way as a screw thread would be cut, using mechanical and electronic equipment of the highest quality and accuracy. Clearly enough, accuracy demands that the movement of the grinding wheel be faultlessly synchronized with that of the dressing tool; currently, this is achieved by adopting either all-mechanical linkages or fluid power controls, which are characterized by ultra-high precision. Accordingly, where drive ratios have to be varied to suit different construction parameters of the work, such as module, number of teeth etc., it becomes necessary to make use of change gears, or at least, to adopt suitable speed control means that permit of effecting the requisite modifications quickly (thus reducing machine down time) in order to adapt the profile of the grinding wheel to the specifications of the work.
The object of the invention is one of freeing a grinding machine from the necessity for mechanical or fluid power control systems as mentioned above, in such a way that, on the one hand, the grinding operation can be rendered more versatile and the variation of drive ratios that accompany a change of work made swifter and more flexible, and on the other, the operation of dressing the grinding tool can be made considerably more accurate (it will be noted that more accurate dressing has a favourable effect on the quality of the ultimate finish); in short, the dressing operation is made more flexible and speeded up by virtue of the fact that it does not rely on change manoeuvres, but rather, on the entry of new data into the unit by which the entire machine is controlled.
Such an object is not achieved, however, simply by using motors in conjunction with a conventional numerical control circuit, since there are numerous problems connected with such an expedient.
To govern the movement of the various machine axes, the control circuits of the type in question incorporate encoders associated with the shafts of the relative motors. Pulses generated by the encoder are read by the circuit, which calculates the exact position of the axes and then transmits the appropriate feed or rotate instruction to the work or the grinding wheel drive, or in the case of dressing, to the grinding wheel or dressing tool drives.
In the particular case of dressing operations, where accuracy is of paramount importance, and the flank of the thread to be dressed is angled, the grinding wheel must be rotated and the dressing tool traversed along its axis exactly together; at the same time, the tool must be fed into the wheel by a further drive.
Rotation of the wheel and axial traverse of the dressing tool are coordinated by feed steps which are determined by the type of transduction elements utilized, hence by the number of pulses generated: the greater the number of pulses, the smaller the steps, hence better approximation and a finer degree of accuracy (clearly, the number of pulses will be associated with a clock pulse of set duration `t`, generated by the NC).
The problem of accuracy derives from the fact that a given input monitoring frequency exists above which the control circuit becomes unable to read the signals correctly, and also, from the fact that the angular velocity of the grinding wheel is markedly different during grinding and dressing operations. The pulse frequency is in fact given by multiplying together the number of pulses emitted per revolution of the motor with which the encoder is associated, and the running speed of the motor. Accordingly, if the number of pulses generated and the speed of the motor are too high and the facility of monitoring the input is lost, precision of the finish can well be unacceptably low for grinding purposes (or it may well happen that an alarm threshold is triggered in the Numerical Control and operation is inhibited altogether). By the same token, it is preferable to operate at the highest pulse frequency obtainable in order to ensure maximum precision (in any event, a frequency invariably higher than the NC is capable of monitoring).